Lunar cataclysm theory gets a boost

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The theory that the 'lunar cataclysm' which formed the surface of the moon, ended with an intense period of bombardment about 4 billion years ago, has received a significant boost through dating of lunar meteorites found on earth.

In the journal Science, researchers from the University of Arizona report that isotopic dating of impact melts in four lunar meteorites collected in Antarctica and the Libyan desert, provided the best data in 30 years with which to examine the lunar cataclysm hypothesis.

Previous analyses of highland rocks collected on Apollo 15,16 and 17 missions suggested widespread asteroid and cometary collisions in a brief period of time in what was called the lunar cataclysm. That event, about 4 billion years ago, would have created the moon's surface.

Planetary plates are formed by an accretion of molecules changing dust to particles during the intense bombardment. There are two theories of what happened - the process was a billion years of impacts, or one than ended with a heightened frequency of impacts 3.9 to 4 billion years ago.

B.A Cohen and colleagues from the University of Arizona say the point of collection for the moon rocks - the nearside equatorial region of the moon - could mean the dating obscured evidence of more recent events. Lunar meteorites provided the new opportunity to test whether the lunar cataclysm was a moon-wide event.

The dating evidence from the meteorites suggest the cataclysm did affect the entire inner solar system, including Mars.

They found 7 to 9 different impact events between 2.76 and 3.92 billion years ago. "The lack of an impact melt older than 3.92 billion years ago supports the concept of a short but intense period of bombardment in the earth-moon system at 3.9 billion years" says Cohen.

Professor Malcolm Walter from Macquarie University's Department of Earth and Planetary Sciences says the research is an important supplement to the critically important data from the Moon rocks.

"Data from the impact craters on the Moon established a clock to date surfaces on other planets. It's the only mechanism we have at the moment for estimating the age of the surface of the planets. This is a substantial refinement to that clock. It's also a major contribution to working out which model of impact history of the solar system is correct."

The Arizona University researchers said at least 17,000 impacts would have occurred on earth at the time of the moon bombardment. "The largest of these probably produced an immense amount of ejecta, temporarily changed the atmosphere and boiled away large quantities of surface water." It coincides with the earliest evidence of life on earth.

"If a swarm of impacters at 3.9 billion years ago returned the earth to a hot and energetic state, the rise or evolution of life on earth could have been affected." They say the effect could have been to destroy existing life, or to deliver precursor molecules.

"Either way, a catastrophic bombardment of the earth moon system affected the origin and evolution of life."